In my opinion first thing in the morning before the sun comes up. The surroundings would have cooled and you do not have to worry about the sun. if this is not possible the next best time would be a few hours after dark. These would be the ideals but with the good camera settings and taking your time you will still pick up problems.
Good luck...
Rory Paul
Independent Level II Thermographer
Barak Systems
www.baraksystems.com